Use only the `strip` target fort stripping the binary.

$(CP) is not set by GNU Make
Seems like [1] is a full list of what GNU Make defines implicitly
[1] https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html

Mistook CPPFLAGS ("C preprocessor flags") for CXXFLAGS ("C++ flags").

This reverts commit 0956b82d33.

Preprocessor flags are passed in CPPFLAGS.
Debian uses it to pass hardening options (-D_FORTIFY_SOURCE)
to the compiler.

- `nnn` is all about navigational convenience so the user doesn't have to type
- the binary size increases due to readline linkage
- alternative workflow: ^G, cd, `nnn`
- readline required using the prompt and the history would stay after quit

This fixes the build with the BSD install command by eschewing the GNU 
specific -t option, which is not portable.

* make Makefile more packaging friendly

* packagecore: run `strip` before installing